In recent years, spatial sound provision from more than two channels has become increasingly popular such as e.g. evidenced by the wide popularity of various surround sounds systems. For example, the increased popularity of home cinema systems has resulted in a surround systems being common in many private homes. However, a problem with conventional surround systems is that they require a high number of separate speakers located at suitable positions.
For example, a conventional Dolby 5.1 surround system requires right and left rear speakers, as well front centre, right and left speakers. In addition, a low frequency subwoofer may be used.
The high number of speakers not only increases cost but also results in reduced practicality and increased inconvenience to users. Accordingly, research has been undertaken in order to generate speaker sets that are suitable for reproducing or emulating surround sound systems but using a reduced number of speaker positions. Such speaker arrays use directional sound transmissions to direct sounds in directions that will result in them reaching the user via reflections from objects in the sound environment. For example, high frequency signals (which tend to provide most of the perceptual directional cues to a listener) can be directed so that they will reach the listener via reflections off of sidewalls thereby providing an impression to the user that the sound originates from the side (or even behind) the listener.
FIG. 1 illustrates an example of a speaker array capable of providing a surround sound experience using fewer speaker boxes than in a conventional surround sound system.
In the system, the speaker array comprises symmetric left and right speaker arrangements with each speaker array comprising three driver units 101-111 each of which is enclosed in an individual cabinet section.
The surround sound system can generate a direct centre signal by providing identical in-phase signals to the left and right speaker arrangement. Furthermore, front right and left sound signals can be generated by supplying individual right and left signals to the right and left speaker arrangement respectfully. In addition, the speaker array allows directional signals to be transmitted in an outwardly sideways direction. These signals may predominantly be high frequency signals that can emulate surrounds speakers by the signals reaching the listener via reflections off of e.g. walls behind or to the side of the listener.
The direction and degree of directionality of the resulting combined signals emitted from the surround sound system as a whole can be controlled by adjusting the phase difference (or equivalently the delay) between the individual signals being provided to the individual drivers 101-111. However, such audio beamforming can be complex and suboptimal and can result in degradations. In order, to assist and reduce the required beamforming, the individual drivers 101-111 are angled in different directions.
Specifically, a speaker array such as that shown in FIG. 1 can be used to provide a directional transmission of signals such that these can be reflected off of objects to provide sideways (or rearwards) directional cues. However, in addition the speaker array is used to generate a notch which is directed towards the assumed listening position. An audio notch corresponds to an area wherein the sound signals from the different drivers are received out of phase. This results in the sound being perceived as diffuse by the listener and no specific directional cues are perceived. Thus, within the notch, a diffuse sound signal is received that the user cannot determine a specific source location for. Such a diffuse signal can provide an improved sound experience and especially can allow a single centrally placed speaker box to provide sound corresponding to side or rear channels of a surround sound signal without these appearing to originate directly from the speaker box. For example, the notch can allow the user to perceive a sound signal even if the audio environment is such that reflected surround sound signals do not reach the listener. Thus, even if the reflected surround sound signals do not reach the listener, the notch can provide the listener with a diffuse non-directional sound signal carrying the corresponding audio. Thus, the provision of the notch in the direction of the listener provides an improved sound perception from a single speaker box and may enhance robustness of the system to variations in the environment in which it is used.
In order to achieve the best compromise between reflected and direct sound, it is advantageous that the outer drivers 101, 103, 107, 109 are angled outwards. However, in order to optimize performance of the notch (and specifically in order to direct the notch inwards towards the assumed listening position), the outer drivers 101, 103, 107, 109 should by preference be angled inwards.
In the system, the two outer speakers 101, 103, 107, 109 of each speaker arrangement are used to generate the reflected signals and the notch, and in order to provide an acceptable trade off between the conflicting requirements, the middle drivers 103, 109 are angled at a first angle relative to the front of the speaker array and the outer drivers 101, 107 are angled outwards relative to the middle drivers 103, 109. Furthermore, a delay is introduced between the drivers to angle the notch further inwards.
Thus, in the speaker array of FIG. 1, one driver 105, 111 (of each arrangement) is angled directly towards the front, a second middle driver 103, 109 is angled outwards at a first angle and a third outer driver 101, 107 is angled further outwards.
However, although this arrangement provides a suitable arrangement for generating directional signals for reflections, audio beamforming processing is still required in order to direct the resulting notches inwards. Thus, if no delay is introduced between the signals to the outer drivers 101, 103, 107, 109, the resulting notch is still angled outwards and a delay of the signal to the middle driver 103, 109 is required in order to angle the notch inwards towards the assumed listening position.
However, a problem in introducing such a delay is that it tends to introduce audible artifacts that reduce the perceived audio quality. Specifically, sidelobes are generated for higher frequencies resulting in sound components being radiated in undesired directions. This tends to diminish the surround sound effect and to introduce some coloration due to comb filtering.
Also, the speaker array arrangements of FIG. 1 impose a strong limitation on the minimum depth of the system. For example, a system using 65 mm drivers easily results in a minimum depth (Y) of the speaker array of ca. 110 mm. This is highly undesirable in many situations. In particular, as speaker arrays such as that of FIG. 1 are often used with flat screen televisions, the increased depth of the speaker array tends to be perceived as highly undesirable by most consumers.
Hence, an improved speaker array arrangement would be advantageous and in particular an arrangement allowing increased flexibility, facilitated implementation, facilitated manufacturing, a reduced physical size, improved notch generation, improved audio quality and/or improved performance would be advantageous.